The present invention relates to a magneto-optical recording medium for use in a magneto-optical memory, a magnetic record display element and so forth and, more particularly, to a magnetic thin film recording medium which has an easy axis of magnetization in a direction perpendicular to the film surface and permits recording of information by forming an inverted magnetic domain of a circular or arbitrary configuration and reading-out of the information through utilization of the magnetic Kerr effect or like magneto-optical effect.
With ferromagnetic thin films having the easy axis of magnetization in a direction perpendicular to the film surface, it is possible to form small inverted magnetic domains which have a polarity reversal from a uniform magnetication polarity in the film surface uniformly magnetized with an S or N pole. By making the presence and absence of the inverted magnetic domains correspond to "1" and "0", respectively, such ferromagnetic thin films can be used as a high density magnetic recording medium. Of such ferromagnetic thin films, thin films which have a large coercive force at a room temperature and has the Curie point or a magnetic compensation temperature relatively close to a room temperature permit recording of information by forming inverted magnetic domains at arbitrary positions with a light beam through utilization of the Curie point or a magnetic compensation temperature, and they are generally used as beam addressable files.
Conventionally known ferromagnetic thin films which have an easy axis of magnetization in a direction perpendicular ular to the film surface and can be used as a beam addressable file are polycrystalline metal thin films represented by MnBi, amorphous metal thin films, such as Gd-Co, Gd-Fe, Tb-Fe, Dy-Fe, etc., and compound single crystal thin films represented by GIG; however, they have such merits and demerits as described below. The polycrystalline metal thin films which utilize the Curie point for writing, represented by MnBi, are excellent in that they have a large coercive force of several kilooersteds at a room temperature, but defective in that they call for large energy for writing because of their high Curie point (T.sub.c =360.degree. C. in MnBi). Furthermore, since polycrystalline metals are used, these thin films must be formed to have a stoichiometric composition and this introduces technical difficulties in their fabrication. The amorphous metal thin films which perform a writing operation through utilization of the magnetic compensation point, such as Gd-Co and Gd-Fe, possess advantages in that they can be formed on an arbitrary substrate since amorphous materials are used, and that their magnetic compensation temperature can be freely controlled to some extent by the addition of a small amount of impurity, but these thin films have a shortcoming such that their coercive force at room temperature is small (300 to 500 Oe), resulting in recorded information being unstable. In addition, it is necessary, for the fabrication of the thin films of such a small coercive force, to control their composition within 1 atom%, and hence these thin films are not easy in terms of manufacture.
The compound single crystal thin films, represented by GIG, have a serious defect of very high manufacturing costs as compared with other thin films.
The amorphous alloy thin films containing 15 to 30 atoms% of Tb or Dy, such as TbFe and DyFe, which have been proposed as new magnetic thin film recording media free from such defects as described above, possess the following merits:
(1) Since they have an easy axis of magnetization in the direction perpendicular to the film surface and have a large coercive force of several kilooersteds at a room temperature, information can be recorded with high density and the recorded information is very stable.
(2) The coercive force is large and magnetic domains of desired configuration can be stored.
(3) Since they have a large coercive force over a wide range of composition and have excellent characteristics as recording media in a wide range of composition, they need not be severely restricted in composition and can be readily fabricated with a good yield rate.
(4) Since the Curie point is as low as 120.degree. C. in the TbFe and 60.degree. C. in the DyFe, the thermal writing operation utilizing the Curie point can be effected with very small energy.
However, the amorphous alloy thin films, such as the Tb and the DyFe, have the following drawbacks: Namely, it is true that a low Curie point permits a writing operation with small energy, but it impairs an signal-to-noise ratio (S/N) in reading out information by light. This is a very serious defect when these recording media are used as a photomagnetic memory.
A GdTbFe ternary amorphous thin film, developed as a solution to this defect, have, to some extent, the advantages of both the TbFe with excellent recording characteristics and the GdFe with excellent optical reproducing characteristics.
These structures are capable of improving to some extent the recording and reproducing characteristics as compared with the single-layer media but they are not satisfactory.